Halo 3 skulls?

[snakeyes789]

do you have to do the campaign on a certain level to get the skull because it won't appear but i know that it is the right place

 

[Bob Dole]

The difficulty of the campaign must be on normal or higher to get the achievements for the skulls. There are two skulls (Thunderstorm and Mythic) that you can get on easy, but will not show up as an achievement if I am not mistaken.

 

[washable mist]

i am pretty sure they will only appear on the hardest difficulty

 

[thacker]

most you have to do on the legendary difficultie.

 

[Brute12]

Make sure you have the difficulty to normal or higher. If you dont the skull won't appear.